Establishment of pattern is a major theme in developmental biology, whether it is studied at the molecular, cellular, or anatomical level. While a great deal of emphasis has been placed on applying the tools of molecular biology and genetics to the study of pattern emergence, the problem of maintenance and repair of pattern has been studied primarily from an anatomical viewpoint. Processes such as limb regeneration (in insects and amphibians) and imaginal disk repair (in insects) are examples of pattern repair. We propose the existence of embryonic Pattern repair system (Prs) genes that are responsible for the detection and the repair of pattern errors during Drosophila melanogaster embryogenesis. We have begun isolating and analyzing mutations that effect embryonic pattern repair. Embryonic pattern repair is observed in experiments where the dosage of the maternal effect gene, bicoid (bcd), is modulated. During oogenesis, bcd mRNA is anchored at the anterior end of the oocyte. Translation of bcd message upon fertilization results in a concentration gradient of BCD protein, which in turn triggers a cascade of transcriptional events that result in the specification of anterior structures (duplication of posterior structures is observed in the absence of BCD activity). Decreasing or increasing the bcd gene dosage from one to four copies, two copies is the normal complement, causes a number of anterior morphological markers to be shifted anteriorly or posteriorly, respectively. One would expect a corresponding size decrease or increase in larval anterior structures or possibly a reduction of viability. Unexpectedly, both of these embryo types develop to produce 'normal' larvae with viability and fertility equal to wild-type. These observations indicate that the embryo has the capacity to compensate for the pattern perturbations caused by changes in the BCD gradient. It is this compensatory response we are attributing to the pattern repair system. When and how does the pattern repair occur? In order to understand how the embryo responds to a pattern defect, one must have a detailed fate map of the affected area that includes information about cellular migration, shape, mitosis, and death. We have developed tools for selectively marking individual cells in live embryos and following their behavior through development. In addition, we and others have developed protocols for monitoring cell death in live embryos. With such a detailed fate map for wild-type embryos, we will be able to determine the changes in cellular behavior during the pattern repair process. What are the genes involved in pattern repair? Pattern repair genes will be identified by taking advantage of the observation that embryos laid by females with six copies of the bcd gene have reduced viability. Therefore, there is a threshold for pattern repair between four and six copies of the bcd gene. A genetic screen for dominant mutations that decrease the viability of embryos that laid by females with four copies of the bcd gene has been initiated and will be extended. Prs mutations will be analyzed at the molecular and cellular levels.